Composites are becoming more widely recognized as a viable solution to a range of applications due to their inherent strength to weight ratio. Composite tubular structures are currently employed in many fields such as the motor sports, marine, aerospace and medical fields. In use, these structures may be subjected to axial tensile and compressive loads as well as bending and torque loadings, for example, when used to provide rods or struts. With increasing demand to transmit working loads directly through to the composite tubular structure comes a requirement also to provide a suitable end fitting or connector arrangement.
A known composite tubular structure and connector arrangement is disclosed in WO 2009/066070. The connector comprises an embedded ovoid screw-threaded nut which creates a bulge in the overlying filaments of the composite member, and an annular wedge for placement over the nut and composite material. The outer surface of the nut produces a region where the filaments flare radially outward and then inward to form the bulge. This provides a tapering surface against which the nut and an annular wedge can be urged to exert a clamping load on the composite material between.
In WO 2009/066070, the ovoid nut can produce residual stresses in the structure during the curing process, as well as having a negative effect on manufacturability due to filament placement and angles. Additionally, if the slope of each side of the nut is at an angle θ relative to a longitudinal axis of the composite tubular structure, then the load being carried by the filaments needs to be transmitted through an angle of 2θ at the point where the angle changes between the two slopes (i.e. at the widest extent of the bulge). This can result in a region where stresses are concentrated and potentially this can limit the overall axial loads (tension/compression) which may be safely carried by the tubular structure.
In Greek Patent Application No. 20150100371, a tubular structure is described which uses the connection system of WO 2009/066070 at one end and a narrower connection system at the other having a radially inward tapered section to retain a nut. The nut for the narrow end of the tubular structure can be fed through from the opposite end which has a larger internal diameter. Weight savings have been made through this arrangement due to one end having a smaller diameter. In addition, while tests have shown that the new connection system offers axial strength benefits, because the deflection angle of the filaments in the tapered section at the narrower end is less, the axial design loads of the tubular structure as a whole are limited by the capabilities of the axially-weaker connection.
U.S. Pat. No. 4,848,957 describes a connection system for a plastic composite tube where a series of acute angled cutouts are provided in the end of the tube to allow the end to be closed down into an inwardly tapered shape once a nut has been introduced. GB-A-2247930 describes a similar arrangement where in place of the acute angled cutouts, an array of axial slots are cut in the end of the tube allowing the parts of the end of the tube to converge, thereby providing inwardly tapered sections at each end. These arrangements would have limited axial load transmission properties due to the way that the ends are formed.
US-A-2012/125146 describes a composite rod having a pair of tapered metal end-pieces that attach to an inner tube. The arrangement is then embedded within wound filaments to form an outer layer which is then polymerised to form the tubular composite structure comprising the inner and outer layers. A rod end can then be screwed into a thread located in a nose of the tapered metal end-piece for connection to another component.
There is a desire to, where possible, reduce the weight of the connector and composite tubular structure, simplify manufacture of the components and design them so they can be used in many different applications.
Further, with increasing demand for lightweight tubular structures, there arises a need to design such structures which can bear the loads more efficiently for a given weight.